Field of the Invention
The present invention relates to a method and a magnetic resonance scanner for generating a data set.
Description of the Prior Art
For magnetic resonance examinations, examination objects are placed in a magnetic field in order to produce, in the examination object, a longitudinal magnetization of nuclei spins in the direction of the external magnetic field, which magnetization can be used for magnetic resonance experiments. Different nuclei can be examined, e.g. hydrogen nuclei (protons), sodium nuclei, carbon nuclei and some others.
The resonant frequencies of the examinable nuclei are different. At a magnetic field strength of 1.5 T, protons have a resonant frequency of approximately 63 MHz, sodium nuclei a resonant frequency of 16 MHz. A differentiation in the resonant frequencies, particularly in the proton resonant frequencies, is additionally caused by the nuclei being in different chemical environments. This frequency shift is also termed “chemical shift”. Without the chemical shift, a magnetic resonance spectrum would have only little diagnostic value. As a result of the chemical shift, the resonant frequencies of the protons of fat and water have a separation, the separation of the dominant fat resonance being about 225 Hz or field-independently about 3.3 ppm at 1.5 T.
However, in magnetic resonance imaging this effect results in “chemical shift artifacts”. For better understanding, the chemical environment is regarded in simplified terms as an additional magnetic field that shifts the resonant frequency. This causes two problems. The spatial encoding has superimposed thereon the read gradient of an additional magnetic field, so viewed as a whole the magnetic field for water protons and fat protons remains different. The additional magnetic field is also once again dependent on the fat in which the protons are bound. However, the differences are less, for which reason a distinction is only made between fat and water in the following description.
For data acquisition with simultaneous switching of a readout gradient, in the image produced from the scan data this resonance shift between water and fat causes the fat signal or fat image to be shifted compared to the water signal or water image.
In addition, for slice selection during which a gradient is switched simultaneously with an RF excitation pulse, this shifting of the resonances causes the slices from which water protons and fat protons are selected to be shifted relative to one another.
A gradient is a non-constant magnetic field which is superimposed on the main magnetic field B0. A gradient is used to make the resonant frequency of the protons spatially dependent.
In the following, the signals or also the protons of water are also termed water signal, water component or water component signal. Fat is similarly designated. Fat suppression means the suppression of the fat component signal.
In order to prevent the chemical shift artifact, it is known to suppress the fat signals. Several methods for achieving fat suppression are known:
One means of fat suppression is spectrally selective suppression. From Bottomley et al., In vivo nuclear magnetic resonance chemical shift imaging by selective irradiation, Proc. Natl. Acad. Sci. USA, Vol. 81, pp. 6856-6860, 1984 it is known to first selectively excite the fat protons using an amplitude-modulated radiofrequency pulse. For this purpose, a gradient must not be switched while the RF pulse is applied. This RF pulse is therefore not slice selective, but only frequency selective. This causes the excited protons to be folded over in the transverse plane. Slice selection is not achieved until a subsequent spin echo sequence in which a slice selective excitation pulse is followed by a refocusing pulse. The disadvantage of this method is that, if the 90° or 180° pulses are sub-optimal, a residual signal of the unwanted component will remain.
A variant is described in Haase et al., 1H NMR chemical shift selective (CHESS) imaging, Phys. Med. Biol., 30(4), pp. 341-344, 1985. After the first, frequency selective 90° excitation pulse, a spoiler gradient is applied that dephases the signal of the fat component that was folded over in the transverse plane by the first RF pulse. How well the subsequent RF pulses are adjusted is therefore immaterial.
For spectrally selective fat suppression, the unwanted component is therefore first excited by a frequency selective 90° RF pulse such that it has no effect on the subsequent experiment. However, slice selection is not possible until thereafter, as no slice selection gradient can be applied during injection of a frequency selective RF pulse.
Another form of fat suppression is that of the inversion methods. These constitute a special case of the inversion recovery (IR) sequences in which the magnetization is initially excited using a 180° pulse, also known as the inversion pulse.
Representative of this type of fat suppression method is the so-called STIR (short TI inversion recovery) sequence, see Bydder and Young, MR Imaging: Clinical Use of the Inversion Recovery Sequence, J Comp Assist Tomogr, 9(4), pp. 659-675, 1985. Here an inversion pulse is injected which folds all the components, i.e. water and fat protons, through 180°. A waiting time which is selected such that the relaxation curve of the fat component passes through the zero crossing is then allowed to elapse. At this point in time the excitation pulse of a spin echo sequence is injected, wherein only the water protons provide a signal contribution, as their signal has no zero crossing. It is considered a disadvantage of this method that the signal of the water protons is also comparatively small at the zero crossing time of the other component. Also the waiting time is relatively long, which means that overall exposure time is increased.
The SPAIR (spectral attenuated inversion recovery) method is also known. In contrast to STIR, the 180° inversion pulse is selectively targeted at the fat protons, which means that the water component signal is not reduced at the zero crossing of the fat. However, the long exposure time remains. An application of this method is described in Lauenstein et al., Evaluation of Optimized Inversion-Recovery Fat-Suppression Techniques for T2-Weighted Abdominal MR Imaging, JMRI, 27(6), pp. 1448-1454, 2008.
A variant of the SPAIR sequence is the SPIR (spectral presaturation with inversion recovery) method. In contrast to SPAIR, the first RF pulse is not a 180° inversion pulse, but a 100° to 110° pulse. This enables the exposure time to be reduced, as the zero crossing of the fat signal is reached more quickly.
An overview study of the fat inversion methods is provided by Ribeiro et al., STIR, SPIR and SPAIR techniques in magnetic resonance of the breast: A comparative study, J. Biomedical Science and Engineering, 6, pp. 395-402, 2013.
Another way of utilizing the different resonant frequencies of the water and fat protons is employed in the Dixon technique named after its inventor, cf. Dixon W. T., Simple Proton Spectroscopic Imaging, Radiology, 153 (1), pp. 189-194, 1984. Here two images are acquired using different echo times, wherein the echo times are selected such that the signal contributions of fat and water add to a maximum in one instance and cancel each other out to a minimum in the next image. These images can be set against one another so as to produce pure water and fat images. However, two images always have to be acquired and prior knowledge concerning the echo time settings is also required.
A procedure differing from the techniques described is constituted by SSGR (slice selective gradient reversal), see Park H W et al., Gradient Reversal Technique and its Applications to Chemical-Shift Related NMR Imaging, Magn. Res. Med., 4, pp. 526-536, 1987. This makes use of the fact that, as described in the introduction, the different resonant frequencies also shift the slices of the fat component and water component relative to one another. In a spin echo sequence having two slice selective RF pulses whose center frequency is tuned to the water frequency, the polarity of the gradient of the refocusing pulse is reversed compared to the polarity of the gradient of the excitation pulse, causing the fat signal to be rephased only partially or not at all and the slice selection gradient to additionally have a dephasing effect on the fat protons. The two RF pulses are not chemically selective.
In order to achieve accelerated image acquisition, the thus obtained spin echo can also be read out using EPI (echo planar imaging), cf. Ivanov D. et al., A Simple Low-SAR Technique for Chemical-Shift Selection with High-Field Spin-Echo Imaging, Magn. Res. Med., 64, pp. 319-326, 2010.
It is also known to use the SSGR method with two refocusing pulses, see Nagy Z. and Weiskopf N., Efficient Fat Suppression by Slice-Selection Gradient Reversal in Twice-Refocused Diffusion Encoding, Magn. Res. Med., 60, pp. 1256-1260, 2008. This is a variant of SE-EPI using two refocusing pulses for diffusion weighting.
The techniques based on gradient reversal (SSGR) always require at least one 180° refocusing pulse.